I will answer your question then I will babble on for a bit.
. The break ware sensor is only on the front nearside mate. The rears do not have it
Babble Babble Babble………….
It is a bit of a fast this break ware sensor stuff!
I have upgraded my discs and pads, the pads are Red Stuff and do not have break ware sensors so I have cable tied the sensor cable up and terminated the connection with electrical tape (so the stupid light on the dash goes away). It is the nearside front that has the sensor and that is it. The fronts obviously ware out quicker than the backs and if you change the pads at the front the backs could need replacing before the sensor is triggered for the front left. Again the ware will be slightly different from left to right any way.
Don’t rely on the sensor mate, get down and have a look to see if there is a decent amount of pad between the metal plate of the pad and the disc.